To compile .cs programs on windows shell, make sure csc.exe is in your Path:

csc program.cs  (produces program.exe)
csc /t:library lib.cs  (produces lib.dll)
csc program.cs /r:lib.dll (compile with dll reference)
program.exe  (runs program)

If using the new .Net Core SDK, consult
https://www.microsoft.com/net/core#windowscmd


For Mono on Mac and Linux:

mcs program.cs  (compiles)
mcs -t:library lib.cs
mcs program.cs -r:lib.dll
mono program.exe  (runs)


Microsoft is also developing its own version of .Net "Core" for Mac, Linux,
but there are currently some problems.